These studies employed similar in vitro models of astrocyte–neuron co-cultures to demonstrate that astrocytes derived from methyl-CpG-binding protein 2 (MeCP2)-lacking mice (a model for Rett Syndrome) (33), astrocytes derived form fragile X mental retardation 1 (FMR1) knock-out mice (a model for Fragile X syndrome) (36), astrocytes pre-treated with ethanol (a model for FASD) (35), and astrocytes derived from human Down syndrome fetuses (34) foster an altered neurite and dendritic spine development in co-cultured rodent hippocampal neurons. This evidence concerns the gene FMR1 and Down syndrome.